172 THE PRINCIPLES OF SCIENCE. 



There can be no doubt that this and other predictions of 

 eclipses attributed to ancient philosophers were due to an 

 obscure knowledge of the Metonic Cycle, a period of 6585 

 days, or 223 lunar months, or about 19 years in which a 

 nearly perfect recurrence of the phases and eclipses of the 

 moon takes place ; but if so, Thales must have had access 

 to a long series of astronomical records either those of the 

 Egyptians or the Chaldeans. There is a well known story 

 as to the happy use which Columbus made of the power 

 of predicting eclipses in overawing the islanders of Jamaica 

 who refused him necessary supplies of food for his fleet. 

 He threatened to deprive them of the moon's light. ' His 

 threat was treated at first with indifference, but when the 

 eclipse actually commenced, the barbarians vied with each 

 other in the production of the necessary supplies for the 

 Spanish fleet.' 



Exactly the same kind of interest and awe which the 

 ancients experienced at the prediction of eclipses, has been 

 felt in modern times concerning the return of comets. 

 Seneca indeed asserted in most distinct and remarkable 

 terms that comets would be found to revolve in periodic 

 orbits and return to sight. The ancient Chaldeans and 

 the Pythagoreans are also said to have entertained a like 

 opinion. But it was not until the age of Newton and 

 Halley that it became possible to calculate the path of a 

 comet in future years. A great comet appeared in 1682, 

 a few years before the first publication of the ' Principia,' 

 and Halley showed that its orbit corresponded with those 

 of remarkable comets rudely recorded to have appeared 

 in the years 1531 and 1607. The intervals of time indeed 

 were not quite equal, but Halley conceived the bold idea 

 that this difference might be due to the disturbing power 

 of Jupiter, near which great planet the comet had passed 

 in the interval 1607-1682. He predicted that the comet 

 would return about the end of 1758 or the beginning of 



